In a laser diffraction-type particle size distribution measuring apparatus, particles in a state of dispersion in a medium are irradiated with a laser beam (measuring light) so that the spatial intensity distribution of the laser beam that has been diffracted and scattered by the particles is detected by photodetector elements. From the results of measurement, a mathematical operation is carried out on the basis of the Fraunhofer diffraction theory or the Mie scattering theory so that the particle size distribution of the particles is calculated. This mathematical operation method on the basis of the Fraunhofer diffraction theory or the Mie scattering theory was obtained assuming that the laser beam is scattered only once by a particle. Therefore, the particle size distribution of particles can be calculated with high precision when the concentration of the particles in the medium is within an appropriate concentration range.
In the case where the concentration of particles in a medium is too high, however, the laser beam for irradiation is scattered by a certain particle so as to be a scattered beam, which is further scattered by another particle. Thus, multiple scatterings take place, which causes a large error between the calculated particle size distribution of the particles and the actual particle size distribution of the particles.
Accordingly, it is necessary to prepare a sample that is thin in the direction of the optical axis of the laser beam in order to reduce the occurrence of multiple scatterings of a beam in the case where the particle size distribution of particles is measured in a sample that contains a high concentration of particles, such as a paste or a slurry.
Here, an example of a laser diffraction-type particle size distribution measuring apparatus is described. FIG. 5 is a schematic diagram showing the structure of a conventional particle size distribution measuring apparatus. FIG. 4 is a schematic diagram showing the structure of an example of a sample cell. In FIG. 5, direction X is a direction that is parallel to the ground, direction Y is the direction that is parallel to the ground and perpendicular to direction X, and direction Z is an upward direction that is perpendicular to direction X and direction Y.
A particle size distribution measuring apparatus 109 is provided with a sample cell 5 in which a sample S is contained, a table (sample cell placing portion) 130 having a laser beam passing hole 130a, an optical system for irradiation 110 having a laser beam source 1, a collimator 2 and a transparent cover 3, an optical system for measurement 120 having a condenser lens 6 and a ring detector (forward diffraction and scattering optical sensor) 7, and a control unit 140 for controlling the entirety of the particle size distribution measuring apparatus 109 (see Patent Document 1).
In the lower portion of the particle size distribution measuring apparatus 109, the laser beam source 1, the collimator 2 and the transparent cover 3 are provided in this order from the bottom as part of the optical system for irradiation 110.
In the middle portion in the upward and downward directions of the particle size distribution measuring apparatus 109, the table 130 is provided and the sample cell 5 is placed on top of the table 130.
In this structure of the optical system for irradiation 110, the laser beam generated by the laser beam source 1 passes through the collimator 2 so as to be a parallel beam with which the sample cell 5 is irradiated when directed in the upward direction (direction Z). Here, the parallel beam has a cross-section in a circular shape that is perpendicular to the optical axis and has an area of approximately 1 cm2. Thus, the laser beam is diffracted and scattered by the particles within the sample cell 5, thereby causing a spatial intensity distribution pattern of the diffracted and scattered beam.
In the upper portion of the particle size distribution measuring apparatus 109, the condenser lens 6 and the ring detector 7 are provided in this order from the bottom as part of the optical system for measurement 120. The ring detector 7 has a number (64, for example) of photodetector elements having a light receiving surface in ring form or semi-ring form having a radius that is different from each other arranged in a concentric form with the optical axis of the condenser lens 6 at the center, where each photodetector element allows a beam having a diffraction or scattering angle in accordance with its respective location to enter. Accordingly, the output signal of each photodetector element represents the intensity of the beam for each diffraction or scattering angle.
In this structure of the optical system for measurement 120, diffracted and scattered beams within 60° relative to the upward direction are condensed onto the light receiving surface of the ring detector 7 via the condenser lens 6 so as to focus into a diffraction and scattering image in ring form.
The sample cell 5 is provided with a transparent glass plate (first substrate) 11 in plate form (thickness: t, width: Y, length: X) and a transparent glass plate (second substrate) 13 in plate form (thickness: t, width: Y, length: X). A recess (recess for measurement) 12 in which a sample S is contained is created in the center portion on the upper surface of the glass plate 11. The recess 12 is in circular form having a cross-sectional area of approximately 1 cm2 as viewed from the top, and the depth of the recess 12 is set at a certain distance Δt (0.1 mm or greater and 0.5 mm or smaller, for example).
When the glass plate 13 is provided so that the lower surface of the glass plate 13 makes contact with the upper surface of the glass plate 11, the distance between the lower surface of the glass plate 13 and the bottom of the recess 12 equals the set distance Δt. Accordingly, this sample cell 5 can make the thickness of the sample S a predetermined thickness Δt, which is thin relative to the optical axis of the laser beam when the sample S is contained in the recess 12, and as a result, the beam can be prevented from scattering a multiple number of times.